In a massive rotary apparatus, such as a rotary mill for grinding ore, a powerful drive system is necessary to initiate and maintain rotation of the apparatus. Starting torques for an apparatus of this type are extremely high. For instance, in autogenous or semi-autogenous mills used for grinding mineral bearing ores and in other large rotary mills such as ball mills, rod mills and cement mills, a rotary mill or drum may have a diameter of the order of 28 to 36 feet and may weigh as much as 1,000 tons. Typically, a mill of this size requires a drive motor having a power output of 10,000 horsepower and above.
In conventional drive systems for mills of this type, power is generally delivered to the mill for rotating the mill about a central axis. This power is delivered to the mill by way of several pinion gears coupled to at least one ring gear connected to the mill. As the mill rotates, the charge in the mill is in the form of a slurry of ore, water and grinding media which tumbles inside the mill and cascades against the inner wear surface portions of the mill. This tumbling action causes the ore to be broken into small pieces and finally reduced in size so that the ore can be taken out of the mill and transported to other areas for further processing.
As the slurried ore and grinding media are tumbled in the mill, tremendous reaction forces are generated which cause the structure of the mill to deform and to be displaced in all directions. This requires the mill itself to be constructed so that it can withstand such large forces which would otherwise seriously limit the operating life of the mill to a relatively short period of time.
It is generally known that gears used with a mill of this type can fail in a relatively short time due to pitting and alignment problems. The gears must be frequently maintained and this requires the mill to be shut down. Thus, the throughput of the mill is correspondingly reduced.
To avoid the use of gears, mills of massive size and inertia have been provided with wrap-around drive means comprised of an electric motor having an annular rotor and an annular stator surrounding the rotor. The rotor itself is coupled in some fashion to the main body of the mill while the stator is fixed to and supported from a means adjacent to the mill. Thus, when the drive means is energized, the mill is rotated in response to the rotation of the rotor relative to the stator. Disclosures relating to a drive means of the wrap-around type are found in the following references: Australian Patent No. 476142; U.S. Pat. No. 1,674,516; German Patent No. 537177; Australian Patent No. 291973; Australian Patent No. 264545; and U.S. Pat. No. 3,272,444.
In all of the foregoing disclosures, there is no adequate means of keeping a substantially uniform air gap between the rotor and the stator of a wrap around motor for a massive rotary mill. Thus, a wrap-around motor for a mill of this size cannot provide that there will be no localized heating of the rotor and stator due to the distortion of the rotor and the mill without distortion of the stator because of the independent support for the stator. As a result, the motor cannot continuously provide the power necessary to drive the mill in an economical manner as a substantially uniform gap is mandatory to maintain the required electrical field.
These problems present a significant drawback with such wrap-around motors as applied to massive structures, such as an ore grinding mill of the type described. The problems arise because the aforesaid driving and reaction forces on such a large mill often cause vibrations and distortion of the mill. As a result, the operating of the wrap-around motor is adversely affected because the rotor of the motor can move toward or away from its stator and change the density of the magnetic flux across the gap between the rotor and stator. This change in magnetic flux density causes unequal torques to be exerted on the mill at different locations thereon. Such unequal torques inhibit the continuous rotation of the mill. The problems further create considerable expense due to the wear and tear on the mill and require continuous maintenance and replacement of parts.
A need therefore exists for improvements in wraparound motors which avoids the aforesaid problems. The present invention fills this need.